Steering from electrochemical denitrification to ammonia synthesis

The removal of nitric oxide is an important environmental issue, as well as a necessary prerequisite for achieving high efficiency of CO2 electroreduction. To this end, the electrocatalytic denitrification is a sustainable route. Herein, we employ reaction phase diagram to analyze the evolution of reaction mechanisms over varying catalysts and study the potential/pH effects over Pd and Cu. We find the low N2 selectivity compared to N2O production, consistent with a set of experiments, is limited fundamentally by two factors. The N2OH* binding is relatively weak over transition metals, resulting in the low rate of as-produced N2O* protonation. The strong correlation of OH* and O* binding energies limits the route of N2O* dissociation. Although the experimental conditions of varying potential, pH and NO pressures can tune the selectivity slightly, which are insufficient to promote N2 selectivity beyond N2O and NH3. A possible solution is to design catalysts with exceptions to break the scaling characters of energies. Alternatively, we propose a reverse route with the target of decentralized ammonia synthesis.

Scheme of the one-dimensional reaction phase diagram (1D-RPD) for (a) the 'path a' consisting of rA and rE, (b) the 'path b' consisting of rB, rC and rD, and (c) the overall (optimal) activity trend with the two paths. Solid lines represent the most energetically difficult steps, namely ΔG-limiting step (rmax). Note: Take Fig. 2 in main text as an example. There are two possible paths over a given catalyst (the marked dashed vertical line in Supplementary Fig. 8) to a product. The 'path a' is consisting of rA and rE. The 'path b' is consisting of rB, rC and rD. The ΔG-limiting step (rmax) for 'path a' (Supplementary Fig. 8a) is rA, while rC is the key step, instead of rB, for the path b ( Supplementary Fig. 8b). The preference for the two paths is justified by the comparison between rA and rC steps. Therefore, the 'path a', with minimum rmax, is the optimal path on the examplified catalyst.
As catalysts changes from one material to another, we can employ descriptors (for example, adsorption energy) to establish a reaction phase diagram (RPD) to study the evolution of reaction mechanism over a series of materials. As shown in Supplementary Fig. 8, the reaction free energies (ΔG) of elementary steps extend from points to lines. The crosspoints between the lines of rmax in different paths will separate RPD into several phases. For instance, as schemed in Supplementary Fig. 8c, the two crosspoints between rA/rE and rC separate the RPD into three regions. On the catalysts in left and right windows, the 'path b' is the favorable path, while the optimal one in middle window is 'path a'. Fig. 9 Scheme of the 1D-RPD for (a) the 'path b' consisting of rB, rC and rD, (b) the 'path c' consisting of rC, rD and rF, and (c) the overlapping of the two paths. Black solid lines represent the shared ΔG-limiting steps. Red and green dotted lines refer to path-selection-determining steps for 'path b' and 'path c', respectively.

Note:
Some pathways share the same ΔG-limiting steps, where the comparison upon the ΔG-limiting energy is not sufficient for selectivity analysis. The path-selection-determining steps beneath ΔG-limiting steps require to be identified and compared. As shown in Supplementary Fig. 9a and b, the rB and rF are identified as the selectivity-determining steps for 'path b' and 'path c', respectively. The crosspoint between rB and rF separates the 1D-RPD ( Supplementary Fig. 9c) into two reaction phases. In the left window, the 'path c' is more preferable with the lower ΔG of rF than that of rB. In contrast, rB has lower ΔG than rF in the right phase, indicating the preference of 'path b'.
Note that the selectivity analysis among N2 and N2O productions was conducted through this strategy discussed in Supplementary Figs. 8 and 9, too. Taking two representative metals (left: Fe, right: Ag) as an example, the selectivity of N2 against N2O was analyzed by energetic comparison of key steps, as shown in Supplementary Table 5. Fig. 10 Schematic reaction mechanism for NH3 production.

Supplementary
Supplementary Fig. 11 The two-dimensional reaction phase diagram (2D-RPD) for NH3 production via (a) path 14, (b) path 17 and (c) the combination of the two paths. The grey dotted lines in (c) are the crosslines between the two paths, which separates the 2D-RPD into three phases (parts). All ΔGRPD-limiting energies are referenced to the same color bar on the right.
Note: As shown in Supplementary Fig. 8, for two individual paths with different shapes, their combination can be divided into three parts (phases). Therefore, the activity map of ammonia production was divided to three parts because the same reason. As shown in Supplementary Fig. 8c, three elementary steps can have two crosspoints (marked as square) in 1D-RPD, which can be extended as two crosslines in 2D-RPD. In Supplementary Fig. 11c, the dashed lines are from the overlapping of the two individual activity maps in Supplementary Fig. 11a  Note: As shown in Supplementary Fig. 28, the most of intermediates involved in eNORR are bonding with metals through N atom, where the most species were described by GadNOH* except for NH2OH*, which was chosen as an independent variable (Supplementary Fig. 12l) due to its importance for hydroxylamine production. The intermediates adsorbed with N and O atoms were described by GadtONNO* (Supplementary Fig. 12m and n). The GadO* was described by GadOH* (Supplementary Fig. 12o). Towards a more accurate description in one-dimensional reaction phase diagrams, their adsorption energies were fitted against GadNOH* in three stages (Supplementary Fig. 12 p-t).
Supplementary Fig. 13 The identification of optimal paths towards N2 in different reactivity regions, such as (a-c) GadNOH* < -1.46 eV, (d-f) GadNOH* from -1.46 to -0.53 eV, and (g-i) GadNOH* > -0.53 eV, by reaction phase diagram (RPD) analysis. The selective key steps for different paths were distinguished by different colors and were highlighted by solid lines in different reaction phases.
Note: According to Fig. 4a and b in the main text, non-N2O* pathways (path 1, 6 and 9) were firstly excluded. In details, paths 1, 6 and 9, belonging to non-N2O* mechanism, yield N2 via the key intermediate HONNOH* (Supplementary Fig. 7), indicating that the formation of HONNOH* is a necessary step for the three paths. However, as shown in Fig. 4a and b, HONNOH* formation (black dashed lines) is always more difficult than N2O* further conversion (blue and red dashed lines) over all studied metals. Hence, here, the further identification of optimal paths in different regions is conducted among p-N2O* and dis-N2O* paths, without discussion about paths 1, 6 and 9. From the comparison shown in Supplementary Fig. 13a, b, and c, it can be determined that the NOH*+NO coupling (blue) is the most favorable way of N-N formation for TMs with GadNOH* < -1.46 eV, on which N2 prefers to yield via N2O* protonation (Fig. 4a). Hence, the optimal path in left window is path11. Note that N2 is produced by N2O* dissociation in reactivity region with GadNOH* > -1.46 eV (Fig. 4b). Hence, from the comparison shown in Supplementary Fig. 13d, e, and f, it can be obtained that the most favorable path is path 4 with N* + NO coupling (black) in the middle window (GadNOH* from -1.46 to -0.53 eV). Finally, when GadNOH* > -0.53 eV, path7 through NO* + NO coupling (red lines in Supplementary Fig. 13g, h, and i) is identified as the optimal one. Supplementary Fig. 24 The thermodynamical analysis between N2O and NH3 selectivity at 0 V vs RHE. The ΔGRPD-limiting steps for N2O and NH3 are shown in red and black solid lines, respectively.

Note:
Cu shows similar ΔGRPD-limiting energies for N2O and NH3 production. For metals with stronger GadNOH* adsorption energies than Cu (in the left of 1D-RPD), the NH3 production has much higher ΔG-limiting energies than N2O due to the strong adsorption of NH2*. For Au and Ag (less reactive than Cu), the protonation of solvated NO (towards NH3 production) is thermodynamically and kinetically difficult than its adsorption (towards dual-N products). This means the most TMs are N2O-selective and Cu exceptionally shows comparable thermodynamic limits for N2O and NH3 productions. Fig. 25 The experimental activity trends for NH3 and N2O productions, adopted from J. Am. Chem. Soc. 144, 1258-1266(2022. From left to right, the metals are Fe, Ni, Co, Pt, Pd, Cu and Ag, respectively.  Cyclic coupling --------- Note that the tag of "Cyclic coupling" means that the formation of N2O* proceeds the following route: